Similar to PR#42360 this commit adds missing properties for
absolute path handling in io.systemd.Unit.StartTransient for
the `Exec` context and a macro helper to share the common code.
The properties added are:
IPCNamespacePath, NetworkNamespacePath, RootDirectory, RootHashPath,
RootHashSignaturePath, RootImage, RootVerity, UserNamespacePath,
RootMStack.
Note that RootHashPath, RootHashSignaturePath need custom apply
functions because the varlink name "RootHashPath" differs from the
name that needs to be written into the unit file ("RootHash=")
and the iovec must be cleared.
Michael Vogt
ab344eb750